patch 8.2.4901: NULL pointer access when using invalid pattern

Problem:    NULL pointer access when using invalid pattern.
Solution:   Check for failed regexp program.

src/testdir/test_buffer.vim

@@ -410,6 +410,13 @@ func Test_buffer_scheme()
   set shellslash&
 endfunc
 
+" this was using a NULL pointer after failing to use the pattern
+func Test_buf_pattern_invalid()
+  vsplit 0000000
+  silent! buf [0--]\&\zs*\zs*e
+  bwipe!
+endfunc
+
 " Test for the 'maxmem' and 'maxmemtot' options
 func Test_buffer_maxmem()
   " use 1KB per buffer and 2KB for all the buffers